Shareholder's Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended			1 Months Ended	9 Months Ended		1 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Aug. 31, 2011 Unit	Aug. 31, 2011 Total Shareholders' Equity Attributable to Assured Guaranty Ltd.	Sep. 30, 2012 Total Shareholders' Equity Attributable to Assured Guaranty Ltd.	Sep. 30, 2011 Total Shareholders' Equity Attributable to Assured Guaranty Ltd.	Aug. 31, 2011 Common Stock	Sep. 30, 2012 Common Stock	Nov. 14, 2011 Common Stock	Aug. 04, 2010 Common Stock
Payments for Repurchase of Common Stock	$ 24	$ 23		$ 23	$ 24	$ 23
Stock Repurchased During Period, Shares							2,000,000	2,066,759
Stock Repurchase Program, Number of Shares Authorized to be Repurchased									5,000,000	2,000,000
Deferred Compensation Units			138,375